The melt extrusion of high molecular weight polymers, for example, hydrocarbon polymers and polyamides, into shaped structures such as tubing, pipe, wire coating or film is accomplished by well-known procedures wherein a rotating screw pushes a viscous polymer melt through an extruder barrel into a die in which the polymer is shaped to the desired form and is then subsequently cooled and solidified into a product having the general shape of the die.
In order to achieve low production costs, it is desirable to extrude the polymer at rapid rates. Higher extrusion rates may be readily obtained by increasing the rate of revolution of the extruder screw. However, this technique is subject to limitations imposed by the viscoelastic properties of the polymer substrate. Thus, at very high extrusion rates an unacceptable amount of thermal decomposition of the polymer can result. Further, extrudates having a rough surface are often obtained which can lead to formation of an undesirable pattern on the surface of the extrudate.
It is desirable, therefore, to find highly efficient means of increasing the extrusion rate, while producing articles having smooth surfaces. Conventional wax-type process aids have been found to reduce bulk viscosity and in some cases improve processing properties. However, the efficiency is marginal and the high levels of additive required often adversely affect other properties.
In Blatz, U.S. Pat. No. 3,125,547, it is disclosed that the use of 0.01-2.0 wt. % of a fluorocarbon polymer that is in a fluid state at the process temperature, such as a fluoroelastomer, will reduce die pressure in extrusions of non-fluorinated polymers such as high and low density polyethylenes and other polyolefins. Further, use of this additive allows significant increase in extrusion rates without melt fracture. Fluoropolymer coats the extrusion die, thus improving flow of the polyolefin.
More recently, improved fluoropolymer process aid compositions have been disclosed in for example, U.S. Pat. Nos. 4,855,360; 5,587,429 and 5,707,569. In these fluoropolymer process aid compositions, a second additive, such as a poly(oxyalkylene) or an ionomer resin, is introduced in order to improve extrusion processability of the non-fluorinated polymer.
Various processing aids that are blends of two or more elastomeric or thermoplastic fluoropolymers have been described in the prior art. U.S. Pat. No. 4,904,735 discloses blends of a tetrafluoroethylene homo- or copolymer (which is a solid at the extrusion temperature) with either a fluoroelastomer having a Tg less than the extrusion temperature, or a crystalline fluoropolymer having a melting point below the extrusion temperature. U.S. Pat. No. 5,106,911 discloses blends of a fluoroelastomer and a crystalline vinylidene fluoride homo- or copolymer wherein the latter has a melting point less than the extrusion temperature.
U.S. Pat. Nos. 6,242,548 B1 and 6,277,919 B1 disclose process aids which are multimodal fluoropolymers. The latter process aids may optionally contain a poly(oxyalkylene) polymer.
WO 02/066544 discloses large particle size fluoropolymer process aids which are more effective than finely dispersed fluoropolymers in improving the extrudability of non-fluorinated melt processable polymers.
The effectiveness of all of the preceding process aids may be adversely affected if the extrudable composition of non-fluorinated melt processable polymer contains particulate mineral additives such as antiblocks, pigments and acid scavengers (e.g. TiO2, talc, hydrotalcites, etc.). The fluoropolymer is adsorbed onto the surface of the particulate, and thus is not available for improving the extrudability of the non-fluorinated melt processable polymer. Effectiveness of the fluoropolymer process aid is especially diminished when mineral particulate and fluoropolymer are both present at high concentrations in a masterbatch (or concentrate). Such masterbatches are typically used in the industry for the feeding of fluoropolymer and mineral particulate to the extruder.